MSK调制解调系统设计的外文翻译

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1、毕业论文外文翻译题 目 MSK调制解调系统的设计 学生姓名 赵 琪 学号 0813024034 所在院(系) 物理与电信工程学院 专业班级 通信工程通信081 指导教师 魏 瑞 2012 年 3 月 15 日一：中文译文调频宽频传输MX589 GMSK调制移键控(msk)0.3 GMSK 1200作者Fred Kostedt, Engineer for MX-COM.James C. Kemerling, Engineer for MX-COM.引言随着计算机的普及，数据传输在当今社会的需求也在不断的增加，进而出现了传输数据的无线链路。二进制数据组成的“一零”和“零一”的过渡，产生了丰富的谐波

2、频谱内容，这并不适合射频传输。因此，数字调制领域已得到了蓬勃的发展。从最近的标准可以看到，如蜂窝数字分组数据（CDPD）和Mobitex*指定高斯滤波最小频移键控（GMSK）的调制方法就是数字调制领域的先进技术。GMSK是一种简单而有效的数字调制的无线数据传输办法。为了使我们更好的理解GMSK数字调制的方法，我们会分析MSK和GMSK的基本理论知识，以及如何运用GMSK数字调制方法来实现CDPD和Mobitex系统。GMSK调制解调器降低了系统的复杂性，从而降低系统的成本。但是，也有一些重要的实施细则需要加以考虑。本文将涵盖其中的一些细节，把重点放在“典型”调频收音机拓扑接口的单芯片中的基带调

3、制解调器的中频/射频部分。背景如果我们看一下傅里叶级数展开的一个数据信号，我们可以看到谐波延伸到无穷远。当这些谐波被总结，他们给它的数据信号急剧转变。因此，一个过滤了的NRZ数据流用来调制射频载波将产生相当大的RF频谱的带宽。当然，有严格的FCC的法规频谱和这种制度，这种使用情况通常被认为是不切实际的。但是，如果我们在开始就移除高次谐波的傅立叶级数（即让数据信号通过一个低通滤器），其过程中的数据将逐步急剧减少。这表明，premodulation过滤是一种在无线数据传输过程中减少被占领的频谱的很有效的方法。除了紧凑的频谱，无线数据调制方案必须在有噪音的情况下能获得良好的误码率（BER）性能。其性

4、能也应该是线性的独立的功率放大设备从而允许使用C类功率放大器。为了满足上述标准，学术领域提出的“数据传输”就是是满载的调制策略。大部分的关于数据位或特殊阶段的相位，频率或振幅的术语。一些较显着的技术列于表1。调制技术 缩写频移键控 FSK多层次的频移键控 MFSK连续相位频移键控 CPFSK最小频移键控 MSK高斯最小频移键控 GMSK驯服调频 TFM相移键控 PSK正交相移键控 QPSK差分正交相移键控 DQPSK差分正交相移键控 DQPSK正交调幅 QAM表1 ：调制格式表1中所列出来的每个调制格式适合特定的应用。一般情况下，计划依赖于两个级别（如QAM调制，QPSK调制），需要有更好的信

5、号信噪比（SNR）的超过两个级别的与计划类似的BER性能。此外，在无线环境里，多层次的计划，通常需要更大的功率放大器，其线性超过两个级别的计划。事实上，GMSK使用两个级别连续相位调制（CPM）的格式广受欢迎和采纳。另一点，其主张是，允许使用C级功率放大器（相对非线性）和数据传输速率接近频道带宽（取决于滤波器的带宽和信道间隔）。GMSK的理论基础在详细讨论GMSK之前，我们需要回顾MSK，从而导出GMSK。MSK是一个连续相位调制的调制方案，即调制载波在任何阶段都没有相位不连续性而且频率的变化发生在载波的零通道。MSK的独特之处是由于一个合乎逻辑的频率0和1之间的关系：逻辑0的频率和逻辑1的频

6、率的不同之处就是总是相当于一半的数据传输速率。换言之，调制指数为0.5的MSK，并定义为m = _f x Twhere,f = |flogic 1 flogic 0|T = 1/bit rate例如，1200比特每秒的MSK数据基带信号可以由1200赫兹的逻辑频率1和1800赫兹逻辑频率0组成（见图1）。图1 ：1200波特率的MSK数据信号;a）NRZ数据，b）的MSK信号基级的MSK，如图1所示，在无线数据传输系统的数据传输速率与relatvely相比有低带宽的频道，这是一个强有力的手段。MX-COM加载装置如MX429和MX469是单芯片解决方案的基带MSK系统，将调制解调电路在一个芯片

7、上。另一种方法实现的MSK调制，可实现直接输入NRZ数据为一个频率调制器且其调制指数为0.5（见图2）。这种做法实质上是相当于基级的MSK。但是，直接的办法是压控振荡器的射频/中频部分，即在基带的MSK的电压频率转换发生在基级。图2 ：直接的MSK调制MSK最根本的问题的是，其频谱并非紧凑的，即不足以实现数据传输速率接近射频频道的带宽。一种频谱的策略的MSK扩展揭示旁瓣远远高于数据速率（见图4）。无线数据传输系统，需要更有效地利用射频频道带宽，这是必要的，以减少MSK上旁瓣的能量。早些时候，我们提出一个简单的手段来减少这一能量，即数据流提交给调制器之前，先经过一低通滤波器（前调制过滤）。预调制

8、低通滤波器必须有一个狭窄的带宽即急剧截止频率和很少的超脉冲响应。这就是高斯滤波器的特点，它有一个个脉冲响应的特点是古典高斯分布（钟形曲线），如图3所示，注意没有通过的或响铃的。图3 ：高斯滤波器脉冲响应BT=0.3和BT=0.5图3描述了脉冲响应的高斯滤波器为BT=0.3和BT=0.5与滤波器的-3dB带宽及数据传输速率有关即：因此，对数据传输速率为9.6kbps和BT为0.3的高斯过滤器-3dB的截止频率是2880Hz。从图3中，注意图中，分散在BT=03的3位期和BT=0.5的2位期上的现象。引起的这种现象称为码间干扰（ISI）。BT=0.3的邻近符号或比特率间的相互干扰比BT=0.5的邻

9、近符号或比特率间的相互干扰更加厉害。当GMSK的参数BT为1时就相当于的MSK。换言之，MSK并不是有意引起码间干扰的。更大的码间干扰使频谱更加紧凑，让解调更加困难。因此，从MSK发展到高斯调制滤波的MSK即考虑到频谱密度的紧凑特点。图4显示了正常化的谱密度的MSK和GMSK。请注意GMSK上减少的旁瓣能源。所以，这意味着GMSK与MSK相比通道间隔在邻近频道干扰处应当更加严格。图4：MSK和GMSK的功率谱密度性能测量GMSK调制器的性能通常是量化的测量信号的信噪比（SNR）和误码率（BER）。与信噪比的有关的Eb/N0，其公式为： WhereS=signal powerR=data rat

10、e in bits per second=noise power spectral density(watts/Hz)=energy per bit最新标准GMSK已经通过了许多的无线数据通信协议。其中具体的两个GMSK调制系统分别是蜂窝数字分组数据（CDPD）和Mobitex。CDPD使用闲置蜂窝传输语音频道数据传输的封闭空气时间蜂窝系统，发送数据速率在19.2kbps且使用参数BT为0.5。由于这种高数据速率，因而促进了30kHz信道间隔的蜂窝网络形成和GMSK的频谱保全。语音优先于数据而且可以将数据传输中断，迫使CDPD系统寻求新的闲置蜂窝通道。这可能被证明是对一个以19.2kbps的数

11、据速率在一个高度拥挤且时间有限的地区执行命令时，对其吞吐量形成的一个障碍。CDPD也将被加入到现有的蜂窝基础设施中，因此，它将会提供广泛的覆盖范围。覆盖范围大和易用性强似乎是CDPD系统的最大的优点。在比预期慢部署CDPD的人有焦虑，也许是有点紧张了它的潜力。与其竞争的专用的数据系统，如Mobitex并非是无足轻重的。虽然Mobitex与比CDPD（8kbps）相比具有较低的数据速率，它也并不与蜂窝语音传输分享其信道。但有几个奥妙之处如这将是使最终用户难以选择最适合其需求的多少实际吞吐量潜力的系统。Mobitex的选择8kbps的数据速率且BT参数设置为0.3，这样使其比CDPD承受了更严格的

12、频道间隔（12.5kHz），但由BT=0.3带来的更大的符号间干扰限制了系统的耐噪声性也让信号产生了失真。狭窄的频带也限制了Mobitex单元间的频偏程度。CDPD和Mobitex的应用采用的对packetting数据的前向纠错技术。图5显示了典型的数据包结构，这两个系统进行比较。前向纠错（FEC）有助于提高系统在信道条件不好时候的吞吐量。图5：CDPD和Mobitex的典型的数据包结构实施的考虑设计一个GMSK调制器/解调器似乎是一个简单的任务。大多数教科书本调制器作为一个“简单”高斯滤波器级联的压控振荡器。然而，在实践中一般并不那么简单。许多章节中一个典型的广播电台，如合成器，IF滤波器，

13、功率amplfier等已远非理想的行为。独特的是，这些合成器给GMSK调制提出了一个独特的问题。由0或1组成的数据模式使频谱响应扩展到附近的直流。大多数频率合成器将不会呼应此低频信号（一个典型的综合有效的高通滤波器特性）。对合成器而言有两种最常见的调制方式将大大有助于在不理想的情况下的实施，即“两点调制”和“正交调制”。两点调制两点调制（见图5）用分裂高斯过滤信号规避这一合成问题；其中一部分是针对振荡器调制输入的，其他部分是用来调节TCXO。TCXO不是频率控制反馈环。因此，TCXO可被低频部分的信号调制，其输出有效地总结了频率合成器里的信号调制压控振荡器。综合信号的频谱响应延伸到直流信号。图

14、6 ：两点调制的无线电框图I和Q的调制正交（I和Q）调制还可以有效地消除合成器的缺点。在I和Q调制中，高斯过滤数据信号分离成同相（I）和正交相位（Q）的组成部分。已调射频信号是由混合的I和Q元件的频率最多的射频载波组成。合成器的作用现在已经减少到仅仅改变载波频率的选择信道上了。达到正交调制的最佳性能的关键在于准确建立I和Q的组件。图7：I和Q电台框图基带I和Q信号可以被用来创建全通相移网络。为了所有频率波段的利益这个网络必须保持I和Q信号间90度的关系。解调解调GMSK信号需要多注意维护一个纯正波形一样调制的信号。选择高斯预调制滤波器主要有三个原因：1) 窄带和急剧截止。2) 较低的超脉冲响应

15、。3) 保存的过滤器输出脉冲地区。第一个条件使GMSK调制的频谱有效率，它也提高了其在解调时的抗噪声性能时。第二个条件使GMSK低相位失真。这是一个重大的关切点，在接收器的信号解调到基带时候，必须注意设计中的IF过滤，以保护这一特点。第三个条件，确保协调一致的信号。当然这是一个相当严格的而且不是物理高斯滤波器容易实现的，一个的相位响应可以保持线形，因此能充分的被的相干解调。在大多数系统上的上述目标的限制还包括：（1） 数据速率（2） 发送滤波器的带宽（BT）的（3） 频道间隔（4） 允许相邻信道的干扰（5） 尖峰载波偏差（6） Tx和Rx载波频率精确度（7） 调制器和解调器的线性度（8） 接收

16、中频滤波器的频率和相位特性。这些制约因素是所有部件的平衡，必须能够提供可靠的GMSK系统。数据传输速率，TX带宽参数BT，峰值载波偏差，和在接收器和发送器之间的载波频率的准确性都是IF滤波器的宽度所必须的。IF滤波器应具有足够的宽度，以适应上述参数中的最大的变化，使接收到的信号将不会进入到滤波器的周围。IF滤波器的周围在较高频率组成的部分接收的数据可以引入过量的群时延（相位失真）。通顺的IF滤波器应该有很少或根本没有群时延，更多群时延的产生，在接收端就会产生错误的数据速率导致误码率（BER）性能的降低。从经验和规则出发，支配的群延迟只有不到10的时间是可以承受的。你高兴的知道要获得这样的性能需

17、要认识其他的一些影响误码率的因素：带宽参数BT，信号强度，衰落等。还可以采取一些相均衡的措施是有助于减少群延迟的，但如果在中频滤波器的设计步骤中得到控制，这些群延迟是可以避免的。CDPD和Mobitex的标准是针对前面提到的两个由MXCOM打造的设备制造的：MX589和MX909。这两个设备被设计成系统的传送和接受基础的接口。MX589是一种能以数据传输速率4kbps-40kbps速率传输以及参数BT为0.5或0.3的多用途的设备。该器件是在一个CMOS进程中实施的，可以在低电源电压（3.0-5.0伏特）和小电流（1.5毫安3.0V）中运作。数字数据接口是一个用于发送和接收的同步串行位。MX9

18、09，也是一个在CMOS进程中实施的，是专门用于Mobitex型系统的，通信的参数为0.3。其数据传输速率可达4kbps-19.2kbps，但应设置为8kbps实现Mobitex的兼容性。它可在电源电压为4.5-5.5伏特和通常的3.0毫安情况下运行。数据接口是一个平行微处理器的I/O兼容总线，而且MX909包含的的所有电路需要执行前向纠错编码和解码的Mobitex格式。基带GMSK信号的解调使用的是高斯型低通滤波器和时钟提取和参考电平补偿电路与数据提取电路。信号的传输部分首先经过高斯低通滤波器相似。信号零交叉参考和时钟频率被提取然后使用峰值检测电路和锁相环去调谐。这一配合努力帮助改善了信号瓣

19、上的噪音。一旦“锁定”那电路中被提取出来的信号将被准确的解调出来。峰值检测电路可以调整直流信号1.5位的变化。这种“夹紧”模式被使用是在载波第一次被接收器检测的时候。该PLL具有宽带收购模式，至少可以在不到8点零交叉参考数时锁定到一个信号。使用这两种方式，让各种设备在接受端感知到载波之后以最快的速度启动数据的解码。两个锁相环及尖峰跟踪电路的模式平常不会工作，一旦初期收购方式获得了锁他们将给予更好的抗噪性。GMSK基带信号的性质要求在直溜系统附近需要有良好的响应就像提到的调制部分一样。更随机数据格局没有大的直流分量，对任何highpass交流电耦合可能需要用于面对的基带信号的GMSK调制器或解调

20、器具有较不敏感的特性。MX589的误码率（BER）性能如图8。此图显示不同设备的highpass在误码率上的特性描绘。Figure 8: BER performance of the MX589这个数字数据从一个以8kbps和带宽参数为0.3以及噪声带宽等于1比特率的静态系统中得到。随着噪声带宽等于比特率，并假设噪声频谱在基带时候是平坦的，X轴在本质是Eb/N0。作为一种替代办法，以完整的DSP实现，这两种设备都提供符合成本效益的解决方案和空间保守的调制解调要求的CDPD和MobitexGMSK系统。总结GMSK提供了一种简单、高效的频谱调制方法的无线数据传输系统（如CDPD和Mobitex）

21、。MX-Com的MX589和MX909的基于GMSK调制解调器提供了单片机解决方案，并协助执行了以GMSK系统为使用标准的调频收音机拓扑。虽然的MX-COM组件集成的基带调制解调器满足了大多数的调制信号处理的需要，但一些关键的系统设计方面确不容忽视。例如，调制器的配置必须有一个平坦光谱响应下降到直流。此外，该接收器的相位响应，必须是线形的横贯那些应当在注意的IF滤波器的数据所占的带宽。按照这些建议再加上单芯片基带调制解调器所保证性能优良的误码率，将会是低能耗和成本低的系统。二：外文资料原文Transmission Wideband FM Modulator msk 1200 MX589 GMS

22、K 0.3 GMSKIntroductionThe proliferation of computers in todays society has increased the demand for transmission of data over wireless links. Binary data, composed of sharp one to zero and zero to one transitions, results in a spectrum rich in harmonic content that is not well suited to RF transmiss

23、ion. Hence, the field of digital modulation has been flourishing. Recent standards such as Cellular Digital Packet Data (CDPD) and Mobitex*specify Gaussian filtered Minimum Shift Keying (GMSK) for their modulation method.GMSK is a simple yet effective approach to digital modulation for wireless data

24、 transmission. To provide a good understanding of GMSK, we will review the basics of MSK and GMSK, as well as how GMSK is implemented in CDPD and Mobitex systems.GMSK modems reduce system complexity, and in turn lower system cost. There are, however, some important implementation details to be consi

25、dered. This paper will cover some of these details, focusing on interfacing a single chip baseband modem to the IF/RF section of a typical FM radio topology.BackgroundIf we look at a Fourier series expansion of a data signal we see harmonics extending to infinity. When these harmonics are summed, th

26、ey give the data signal its sharp transitions. Hence, an unfiltered NRZ data stream used to modulate an RF carrier will produce an RF spectrum of considerable bandwidth. Of course, the FCC has strict regulations about spectrum usage and such a system is generally considered impractical. But if we st

27、art to remove the high frequency harmonics from the Fourier series (i.e. pass the data signal through a lowpass filter), the transitions in the data will become progressively less sharp. This suggests that premodulation filtering is an effective method for reducing the occupied spectrum for wireless

28、 data transmission. In addition to a compact spectrum, a wireless data modulation scheme must have good bit error rate (BER) performance under noisy conditions. Its performance should also be independent of power amplifier linearity to allow the use of class C power amplifiers.The academic field of

29、“Data Transmission” is loaded with modulation strategies that attempt to meet the above criteria. Most involve translation of data bits or patterns into a particular combination of phase,frequency or amplitude. Some of the more notable techniques are listed in Table 1. MODULATION TECHNIQUE COMMON AC

30、RONYMFrequency Shift Keying FSKMulti-level Frequency Shift Keying MFSKContinuous Phase Frequency Shift Keying CPFSKMinimum Shift Keying MSKGaussian Minimum Shift Keying GMSKTamed Frequency Modulation TFMPhase Shift Keying PSKQuadrature Phase Shift Keying QPSKDifferential Quadrature Phase Shift Keyin

31、g DQPSKPi/4 Differential Quadrature Phase Shift Keying Pi/4 DQPSKQuadrature Amplitude Modulation QAMTable 1: Modulation FormatsEach of the modulation formats listed in Table 1 is suited to specific applications. In general, schemes that rely on more than two levels (e.g. QAM, QPSK) require better si

32、gnal to noise ratios (SNR) than two-level schemes for similar BER performance. Additionally, in a wireless environment, multi-level schemes generally require greater power amplifier linearity than two-level schemes. The fact that GMSK uses a two-level continuous phase modulation (CPM) format has con

33、tributed to its popularity. Another point in its favor is that it allows the use of class C power amplifiers (relatively non-linear) and data rates approaching the channel BW (dependent on filter bandwidth and channel spacing).GMSK BasicsPrior to discussing GMSK in detail we need to review MSK, from

34、 which GMSK is derived. MSK is a continuous phase modulation scheme where the modulated carrier contains no phase discontinuities and frequency changes occur at the carrier zero crossings. MSK is unique due to the relationship between the frequency of a logical zero and one: the difference between t

35、he frequency of a logical zero and a logical one is always equal to half the data rate. In other words, the modulation index is 0.5 for MSK, and is defined asm = _f x Twhere, f = |flogic 1 flogic 0|T = 1/bit rateFor example, a 1200 bit per second baseband MSK data signal could be composed of 1200 Hz

36、 and 1800 Hz frequencies for a logical one and zero respectively (see Figure 1).Figure 1: 1200 baud MSK data signal; a) NRZ data, b) MSK signal.Baseband MSK, as shown in Figure 1, is a robust means of transmitting data in wireless systems where the data rate is relatvely low compared to the channel

37、BW. MX-COM devices such as the MX429 and MX469 are single chip solutions for baseband MSK systems, incorporating modulation and demodulation circuitry on a single chip.An alternative method for generating MSK modulation can be realized by directly injecting NRZ data into a frequency modulator with i

38、ts modulation index set for 0.5 (see Figure 2). This approach is essentially equivalent to baseband MSK. However, in the direct approach the VCO is part of the RF/IF section, whereas in baseband MSK the voltage to frequency conversion takes place at baseband.Figure 2: Direct MSK modulationThe fundam

39、ental problem with MSK is that the spectrum is not compact enough to realize data rates approaching the RF channel BW. A plot of the spectrum for MSK reveals sidelobes extending well above the data rate (see Figure 4). For wireless data transmission systems which require more efficient use of the RF

40、 channel BW, it is necessary to reduce the energy of the MSK upper sidelobes. Earlier we stated that a straightforward means of reducing this energy is lowpass filtering the data stream prior to presenting it to the modulator (pre-modulation filtering). The pre-modulation lowpass filter must have a

41、narrow BW with a sharp cutoff frequency and very little overshoot in its impulse response. This is where the Gaussian filter characteristic comes in. It has an impulse response characterized by a classical Gaussian distribution (bell shaped curve), as shown in Figure 3. Notice the absence of oversho

42、ot or ringing.Figure 3: Gausssian filter impluse response for BT = 0.3 and BT = 0.5Figure 3 depicts the impulse response of a Gaussian filter for BT = 0.3 and 0.5. BT is related to the filters -3dB BW and data rate byHence, for a data rate of 9.6 kbps and a BT of 0.3, the filters -3dB cutoff frequen

43、cy is 2880Hz.Still referring to Figure 3, notice that a bit is spread over approximately 3 bit periods for BT=0.3 and two bit periods for BT=0.5. This gives rise to a phenomena called inter-symbol interference (ISI). For BT=0.3 adjacent symbols or bits will interfere with each other more than for BT

44、=0.5. GMSK with BT=_ is equivalent to MSK. In other words, MSK does not intentionally introduce ISI. Greater ISI allows the spectrum to be more compact, making demodulation more difficult. Hence, spectral compactness is the primary trade-off in going from MSK to Gaussian pre-modulation filtered MSK.

45、 Figure 4 displays the normalized spectral densities for MSK and GMSK. Notice the reduced sidelobe energy for GMSK. Utlimately, this means channel spacing can be tighter for GMSK when compared to MSK for the same adjacent channel interference.Figure 4: Spectral density for MSK and GMSKPerformance Me

46、asurementsThe performance of a GMSK modem is generally quantified by measurement of the signal-to-noise ratio (SNR) versus BER. SNR is related to Eb/N0 byWhereS=signal powerR=data rate in bits per second=noise power spectral density(watts/Hz)=energy per bitRecent StandardsGMSK has been adopted by ma

47、ny wireless data communication protocols. Two of the systems specifying GMSK modulation are Cellular Digital Packet Data (CDPD) and Mobitex.CDPD uses the dead air time on cellular systems by sending data packets on idle cellular voice channels.Data is transmitted at 19.2kbps using a BT of 0.5. This

48、high data rate is facilitated by the 30kHz channel spacing of the cellular network and the spectral conservation of GMSK. Voice has priority over data and will interrupt data transmission, forcing the CDPD system to seek a new idle cellular channel. This could prove to be an obstacle to the throughp

49、ut promised by its 19.2kbps data rate when implemented in a highly congested area where dead time is limited.CDPD is being added to the existing cellular infrastructure and therefore promises to offer widespread coverage. The coverage and ease of adaptation appear to be the greatest strengths of the

50、 CDPD system. The slower-than-expected deployment of CDPD has many people anxious and perhaps a bit nervous about its potential.Competition from dedicated data systems such as Mobitex is not insignificant. While Mobitex has a lower data rate than CDPD (8kbps), it is not sharing its channels with cel

51、lular voice transmissions. Several subtleties such as this will make it more difficult for end users to select the system best suited to their needs by obscuring the actual throughput potential of the systems. Mobitexs choice of 8kbps and a BT of 0.3 afford it a much tighter channel spacing (12.5kHz

52、) than CDPD, but the greater inter-symbol interference for BT=0.3 limits the systems tolerance to noise and distortion. The narrower channel also limits Mobitexs tolerance to frequency offsets between units.Both CDPD and Mobitex employ forward error correction in their packetting of data. Figure 5 s

53、hows the typical packet structures of these two systems for comparison. Forward error correction (FEC) helps improve the systems throughput when less than ideal channel conditions exist.Figure 5: Typical packet structures for CDPD and MobitexImplementation ConsiderationsThe design of a GMSK modulato

54、r/demodulator appears to be a straightforward task. Most textbooks present the modulator as a “simple” Gaussian filter cascaded with a VCO. However, in practice it is generally not that simple. Many of the sections in a typical radio such as the synthesizer, IF filter, power amplfier, etc. have far

55、from ideal behavior. In particular, the synthesizer presents a unique problem for GMSK modulation. Data patterns consisting of several consectutive ones or zeros have a spectral response extending down to near DC. Most frequency synthesizers will not respond to this low frequency signal (a typical s

56、ynthesizer effectively has a highpass filter characteristic).Two of the most common modulation methods, which help considerably where the non-ideal behavior of the synthesizer is concerned, are Two-point modulation and Quadrature modulation.Two point modulationTwo point modulation (see Figure 5) cir

57、cumvents this synthesizer problem by splitting the Gaussian filtered signal; one portion is directed to the VCO modulation input, the other portion is used to modulate the TCXO.The TCXO is not in the frequency control feedback loop. Hence, the TCXO can be modulated by the low frequency portion of th

58、e signal, and its output is effectively summed with the signal modulating the VCO in the synthesizer. The composite signal has a spectral response extending down to DC.Figure 6: Two point modulation radio block diagramI and Q modulationQuadrature (I and Q) modulation can also be effective in elimina

59、ting synthesizer shortcomings. In I and Q modulation, the Gaussian filtered data signal is separated into in-phase (I) and quadrature phase (Q) components. The modulated RF signal is created by mixing the I and Q components up to the frequency of the RF carrier, where they are summed together. The r

60、ole of the synthesizer has now been reduced to merely changing carrier frequency for channel selection. The key to optimum performance with quadrature modulation is accurate creation of the I and Q components.Figure 7: I and Q radio block diagramBaseband I and Q signals can be created by using an al

61、l-pass phase shifting network. This network must maintain a 90 degree phase relationship between the I and Q signals for all frequencies in the band of interest.DemodulationDemodulation of the GMSK signal requires as much attention to the preservation of an unadulterated wave form as does modulation

62、 of the signal. The choice of a Gaussian shaped pre-modulation filter was made for three main reasons1:1) narrow bandwidth and sharp cutoff2) lower overshoot impulse response3) preservation of the filter output pulse area.The first condition gives GMSK modulation its spectral efficiency. It also improves its noise immunity when demodulating. The second condition affords GMSK low phase distortion. This is a major concern when the receiver is demodulating the signal down to baseband, and care must be taken in the design of the IF filtering to